Modern data communications technologies have greatly expanded the ability to communicate large amounts of data over many types of communications facilities. This explosion in communications capability not only permits the communications of large databases, but has also enabled the digital communications of audio and video content. This high bandwidth communication is now carried out over a variety of facilities, including telephone lines (fiber optic as well as twisted-pair), coaxial cable such as supported by cable television service providers, dedicated network cabling within an office or home location, satellite links, and wireless telephony.
Each of these conventional communications facilities involves certain limitations in their deployment. In the case of communications over the telephone network, high-speed data transmission, such as that provided by digital subscriber line (DSL) services, must be carried out at a specific frequency range to not interfere with voice traffic, and is currently limited in the distance that such high-frequency communications can travel. Of course, communications over “wired” networks, including the telephone network, cable network, or dedicated network, requires the running of the physical wires among the locations to be served. This physical installation and maintenance is costly, as well as limiting to the user of the communications network.
Wireless communication facilities of course overcome the limitation of physical wires and cabling, and provide great flexibility to the user. Conventional wireless technologies involve their own limitations, however. For example, in the case of wireless telephony, the frequencies at which communications may be carried out are regulated and controlled. Furthermore, current wireless telephone communication of large data blocks, such as video, is prohibitively expensive, considering the per-unit-time charges for wireless services. Additionally, wireless telephone communications are subject to interference among the various users within the nearby area. Radio frequency data communication must also be carried out within specified frequencies, and is also vulnerable to interference from other transmissions. Satellite transmission is also currently expensive, particularly for bi-directional communications (i.e., beyond the passive reception of television programming).
A relatively new technology that has been proposed for data communications is the optical wireless network. According to this approach, data is transmitted by way of modulation of a light beam, in much the same manner as in the case of fiber optic telephone communications. A photoreceiver receives the modulated light, and demodulates the signal to retrieve the data. As opposed to fiber optic-based optical communications, however, this approach does not use a physical wire for transmission of the light signal. In the case of directed optical communications, a line-of-sight relationship between the transmitter and the receiver permits a modulated light beam, such as that produced by a laser, to travel without the waveguide of the fiber optic.
It is contemplated that the optical wireless network according to this approach will provide numerous important advantages. First, high frequency light can provide high bandwidth, for example ranging from on the order of 100 Mbps to several Gbps, using conventional technology. This high bandwidth need not be shared among users, when carried out over line-of-sight optical communications between transmitters and receivers. Without the other users on the link, of course, the bandwidth is not limited by interference from other users, as in the case of wireless telephony. Modulation can also be quite simple, as compared with multiple-user communications that require time or code multiplexing of multiple communications. Bi-directional communication can also be readily carried out according to this technology. Finally, optical frequencies are not currently regulated, and as such no licensing is required for the deployment of extra-premises networks.
These attributes of optical wireless networks make this technology attractive both for local networks within a building, and also for external networks. Indeed, it is contemplated that optical wireless communications may be useful in data communication within a room, such as for communicating video signals from a computer to a display device, such as a video projector.
It will be apparent to those skilled in the art having reference to this specification that the ability to correctly aim the transmitted light beam to the receiver is of importance in this technology. Particularly for laser-generated collimated beams, which can have quite small spot sizes (i.e. cross sectional area), the reliability and signal-to-noise ratio of the transmitted signal are degraded if the aim of the transmitting beam strays from the optimum point at the receiver. Especially considering that many contemplated applications of this technology are in connection with equipment that will not be precisely located, or that may move over time, the need exists to precisely aim and controllably adjust the aim of the light beam.
Co-pending application Ser. No. 09/310,284, filed May 12, 1999, entitled “Optical Switching Apparatus”, commonly assigned herewith and incorporated herein by this reference, discloses a micro-mirror assembly for directing a light beam in an optical switching apparatus. The micro-mirror reflects the light beam in a manner that may be precisely controlled by electrical signals. The micro-mirror assembly includes a silicon mirror capable of rotating in two axes. One or more small magnets are attached to the micro-mirror itself; a set of four coil drivers are arranged in quadrants, and are current-controlled to attract or repel the micro-mirror magnets as desired, to tilt the micro-mirror in the desired direction.
Because the directed light beam, or laser beam, has an extremely small spot size, precise positioning of the mirror to aim the beam at the desired receiver is essential in establishing communication. This precision positioning is contemplated to be accomplished by way of calibration and feedback, so that the mirror is able to sense its position and make corrections.
Co-pending patent application Ser. No. 09/620,943 entitled “Optical Wireless Link,” commonly assigned herewith and incorporated herein by reference, discloses one approach to providing a feedback signal from the receiver to the transmitter over a secondary link. As disclosed in the application, the feedback and control signals are transmitted over a low bandwidth link, such as a radio frequency (RF) link or a twisted pair or similar physical link.
Another approach to providing a light beam alignment feedback signal to the transmitter is disclosed in co-pending patent application Ser. No. 60/234,081 entitled “Optical Wireless Networking with Direct Beam Pointing,” commonly assigned herewith and incorporated herein by reference. In that application, alignment feedback is provided passively by a receiver lens surrounded by a retro-reflective annulus.
As optical wireless links become more prevalent, users will demand greater autonomy in the devices as they are deployed in networks and real-world environments. One area of autonomy that will be expected of such devices is the ability to automatically acquire the signal of a remote link in order to establish the optical wireless communication channel between them. Therefore, a need exists in the art for an optical wireless link that can automatically acquire a line-of-sight communication with another optical wireless link.